It is generally known to reinforce concrete structures with metal elements, such as steel fibers, to give the structures the required mechanical properties.
As bare steel elements may suffer from corrosion, galvanized steel elements have been proposed to give the steel elements a long term corrosion resistance. Galvanized reinforcing steel elements are especially useful for the reinforcement of concrete for construction purposes whereby the reinforced concrete will be exposed to the weather before construction begins, as for example in prefabrication construction.
However, the use of galvanized steel elements in concrete is creating problems: during hardening of the concrete, the galvanized surface of the steel elements will react with the alkaline environment of the concrete to form zinc salts accompanied by hydrogen gas evolution. This is an effect that appears when a strong electronegative element like zinc, aluminum or magnesium is exposed to water. The element has an open circuit potential as defined in the standard ASTM G 15-93. At high pH values, the open circuit potential drops below the hydrogen evolution potential and hence initiates the reduction of hydrogen ions resulting in hydrogen gas evolution.
Hydrogen gas evolution leads to strength and durability problems, as well as to aesthetical problems. Due to hydrogen gas evolution at the interface of the metal elements and the concrete, the bond strength between the metal elements and the concrete is reduced. This is resulting in a reduction of the strength of the reinforced concrete. The durability problem is the result of the reduction in the thickness of the alloy coating due to the reaction of the alloy coating in the alkaline environment. The alloy coating may e.g. be zinc, aluminum or magnesium.
The problems of galvanized steel elements in concrete are described in “Effect of chemical-physical interaction between galvanized steel fibres and concrete”, T. Belleze, R. Fratesi, C. Failla, 6th RILEM Symposium on Fibre-Reinforced Concretes (FRC) BEFIB 2004, 20-22 Sep. 2004, 239-248.
To prevent hydrogen gas evolution the surfaces of galvanized steel elements can be passivated. This can be realised by treating the galvanized steel elements with a chromium based compound. Also the chromate naturally present in the concrete can be sufficient to protect the galvanized steel elements.
However, in recent years it has been recognized that hexavalent chromium raises serious environmental and health problems. Consequently, strict restrictions have been placed on the quantity of hexavalent chromium used in a number of industrial processes and products as for example cement and concrete.
Other attempts to protect galvanised steel comprise the application of an epoxy coating on the galvanised steel. The use of galvanised steel coated with an epoxy coating to reinforce concrete is for example described in JP 53-078625. However, epoxy coatings have serious drawbacks as epoxy coatings act a barrier against a corrosive environment. If there are defects in the epoxy coating through which aggressive agents can penetrate the barrier, corrosion will concentrate on these areas. Local corrosion could make the steel element break where the coating is damaged. Integrity of the epoxy coating is therefore essential as the film must be free from pores, cracks and damaged areas. Epoxy coatings are fragile. Epoxy coated metal elements must therefore be handled with a lot of care during storing, transport and handling. Consequently, as the mixing or installation of the reinforcement elements in the concrete is a rough operation whereby local damages on the surface of reinforcement elements are unavoidable, the use of epoxy coated metal elements or other compounds applied at the surface for the reinforcement of concrete is not a good option.
Many corrosion inhibitors known in the art such as phosphates, silicates, silanes, carbonates and carbonic acids, sulfides and mercaptoderivates, amines and sulfonates have been tested. However, these inhibitors did not give an adequate result as they were not able to avoid hydrogen gas evolution.
Therefore, obtaining an adequate protection of steel elements, and coated metal elements in general, without using chromium compounds and not requiring a 100% closed barrier coating remains to be a problem and efficient solutions are still needed.
WO2006067095A1 by Applicant describes a reinforced structure comprising a cementitious matrix and zinc coated metal elements, wherein said reinforced structure is treated at the interface of said zinc coated metal elements and said cementitious matrix with a compound giving said zinc coated metal element cathodic protection. The compound is selected from the group consisting of the imidazoles, the triazoles and the tetrazoles, whereby said imidazole comprises benzimidazole (BZI).
However, a sufficient amount of compound needs to be applied to the surface to be effective against hydrogen gas evolution. Not only is the process for applying a sufficient amount of compound to the surface of e.g. a zinc coated metal element industrially very difficult, said elements must also be handled with a lot of care during storing, transport and handling to prevent damage to the surface.